As known in the art, high selectivity catalysts (HSCs) for the epoxidation of ethylene refer to those catalysts that possess selectivity values higher than high activity catalysts (HACs) used for the same purpose. Both types of catalysts include silver as the active catalytic component on a refractory support (i.e., “carrier”, such as alumina). Typically, one or more promoters are included in the catalyst to improve or adjust properties of the catalyst, such as selectivity.
Generally, HSCs achieve the higher selectivity (typically, in excess of 87 mole %) by incorporation of rhenium as a promoter. Typically, one or more additional promoters selected from alkali metals (e.g., cesium), alkaline earth metals (e.g., strontium), transition metals (e.g., tungsten compounds), and main group elements (e.g., sulfur and/or halide compounds) are also included.
There are also ethylene epoxidation catalysts that may not possess the selectivity values typically associated with HSCs, although the selectivity values are improved over HACs. These types of catalysts can also be considered within the class of HSCs, or alternatively, they can be considered to belong to a separate class, e.g., “medium selectivity catalysts” or “MSCs.” These types of catalysts may exhibit selectivities of at least 83 mole % and up to 87 mole %. In contrast to HSCs and MSCs, HACs are ethylene epoxidation catalysts that generally do not include rhenium, and for this reason, do not provide the selectivity values of HSCs or MSCs. Typically, HACs include cesium (Cs) as the only promoter.
For all of these types of catalysts, there remains a need to improve the activity and selectivity performance. Moreover, it is well known that with use of a catalyst, the catalyst will age (i.e., degrade) until use of the catalyst is no longer practical, i.e., when activity and selectivity values diminish to a level that is no longer industrially efficient or economical. Thus, there is a further continuous need to extend the useful lifetime (i.e., “longevity” or “usable life”) of these catalysts by maintaining an effective level of activity and selectivity characteristics. The useful lifetime of the catalyst is directly dependent on the stability of the catalyst. As used herein, the “useful lifetime” is the time period for which a catalyst can be used until one or more functional parameters, such as selectivity or activity, degrade to such a level that use of the catalyst becomes impractical. Although many approaches for boosting the activity, selectivity, or stability of the catalyst have been undertaken, there remains a need for further improvements and a more straight-forward and cost-effective method for achieving such an improved catalyst.